Vehicle number plates with a panel onto whose front a retro-reflective film is laminated have long been known. The use of retro-reflective films for vehicle number plates or information signs is required in a multitude of countries by their respective national registration regulations. Due to their retro-reflectivity, retro-reflective number plates or information signs are highly visible under retro-reflective conditions.
Provisions for retro-reflective materials for traffic safety and photometric requirements for reflective substances are laid down, for example, in DIN 67520. Here, retro-reflective materials are classified according to the luminance that a road user perceives from an observed traffic sign. In this case, the construction of the observed retro-reflective structures is irrelevant. In this case, the reflection classes (RA classes) describe the minimum requirements for a material with respect to a specific reflective value.
The requirements for reflective substances for vehicle number plates and traffic signs have increased in recent years. Furthermore, it is presumed that these requirements are raised further in the future. Here, the decreasing visual acuity of a growing number of older drivers figures prominently, the necessity that traffic signs have to be recognizable in the increased urban ambient brightness, and the rise in overhead installations of signs that can hardly be reached anymore by the focused light beam of modern vehicle headlights. These are but a few examples that are supposed to show that the requirements for reflection films used in future have a great need for high-performance retro-reflective layers, as well as a need for large areas of application.
Based on their photometric properties, reflective films are divided into various reflection classes. They are characteristic for the requirements with respect to the specific reflective value (RA), which is a determining factor in the selection of the reflective film. Depending on the different designs, a distinction is made between different types with respect to the reflective film structure.
Some types of reflective films in this case comprise a reflective base layer and glass spheres. In this case, a transparent intermediate layer is provided between the reflective layer and the glass spheres, so that the spheres act as thick lenses and a light beam incident upon the sphere is refracted such that its focus is situated just behind the sphere, substantially on the reflective layer. Thus, light is more or less reflected back to the light source.
Other types of reflective film constitute a more intensively retro-reflective film based on encapsulated glass spheres. In this case, a reflective layer is directly adjacent, in one area, to the glass spheres.
Still other types of reflective film have a reflective layer within the film, based on micro-prism technology. In the case of these films, a layer with a prismatic structure is applied to a substrate layer which, towards the reflective side, is also coated with a cover layer.
It is furthermore known from prior art that, in the case of road markings, very fine glass spheres are applied to the still-moist white marking paint and partially embedded therein. An improved reflection in the case of irradiation is thus obtained in order to increase the visibility of the markings if irradiated.
It is also known that an internal total reflection provides a largely loss-free reflection, so that very high reflective values are achieved by means of this effect. Due to the occurrence of this effect in the interior of a transparent body at the interface with a different, optically thinner medium, it is not necessary to separately provide or manufacture any mirror surfaces. Further more, the problem that the reflectance of the mirror surfaces can be affected by external influences, such as dirt, does not arise.
Furthermore, security features, e.g. in the form of image markings, are known from the prior art. Security features are characteristic properties that prove the authenticity of an item and make falsification impossible, or at least render it considerably more difficult. Such security features are often applied to the items to be marked in the form of optically perceptible image markings, e.g. watermarks or holograms. Providing and manufacturing such security features frequently requires much effort, so that there is generally a need for providing reliable security features whose structure and manufacture are simple and cost-effective.